Diffusion-controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia |
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| Authors: | J R ASHWORTH V S SHEPLEV N A BRYXINA V Yu KOLOBOV & V V REVERDATTO |
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| Institution: | School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT , UK (j;), United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian Academy of Science, Universitetsky pr. 3, Novosibirsk 90, 630090 Russia |
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| Abstract: | Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2O3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8 kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4 kbar. The probable P–T conditions of reaction were in the range 650–790 °C, 8–10 kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10?23 to 5×10?20 m2 s?1. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion. |
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| Keywords: | affinity coronas diffusion granulite reaction mechanisms |
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